1. Field of the Invention
The present invention relates to sound absorbing liners, and particularly to acoustic linings for gas turbine engines for absorbing noises in the low and mid-range frequencies.
2. Description of the Prior Art
Some conventional absorption liners presently utilized in gas turbine exhaust ducts are based on the Helmholtz resonator principle. Examples of such applications are illustrated in U.S. Pat. No. 4,100,993, E. Feder, issued July 18, 1978, which illustrates a deep backing cavity with a narrow slotted opening with a straight depending lip forming an air column at the throat for increasing the mass reactance thereof. In U.S. Pat. No. 4,150,732, R. G. Hoch et al, issued Apr. 24, 1979, a further modification was suggested which includes a flange extending into the cavity from the depending throat-forming lip in order to form a bent duct within the cavity and thus increase the effective length of the inlet duct or inlet passage. If either the effective length of a Helmholtz resonator duct or the volume of the cavity is increased, the resonance frequency can be decreased as can be seen from the following relationship generally attributed to Helmholtz resonators. ##EQU1## where fr is the resonance frequency, S is the cross-sectional area of the duct, l' is the effective length of the inlet duct, and V is the volume of the cavity.
However, in a turbine engine environment, large or deep cavities or vents are, of course, not practical, and thus different configurations such as described in the Hoch et al patent have been contemplated. As shown in the abovementioned patents, the suppressors form part of an acoustic liner within the housing of an engine such as in the exhaust duct of the engine. Thus, a usual configuration of such liners is of two concentric shells with the innermost shell being provided with openings such as slits and the cavity compartments being formed between the concentric shells.